As a synthetic-resin tube issues from an extruder it is frequently passed over a shaping mandrel having an external shape corresponding to the internal shape to be imparted to the workpiece. Such an arrangement is also used with U-shaped, T-shaped, and flanged synthetic-resin workpieces. The mandrel is secured to the extruder so that it is fixed while the synthetic-resin workpiece passes over it longitudinally.
This calibrating mandrel is usually hollow so that water or the like flow through it to cool it and aid in the hardening of the workpiece. Since the workpiece shrinks as it cools and hardens it is frequently considered unnecessary to press it against the outside of the calibrating mandrel. Such an arrangement gives unfortunately inadequate results, however, especially when working with large-diameter tubing.
It is also known to calibrate the outside of a hollow workpiece by means of a so called vacuum-sizing method. In this system the workpiece is drawn outwardly against a die by means of a pressure differential created between the inside and the outside of the tube. Thus the face of the die turned toward the workpiece is provided with a plurality of suction openings through which air or other fluid is drawn so as to pull the workpiece wall outwardly snugly against the die. This arrangement produces a relatively accurate shape on the outside of the tubing.
In none of the known methods is it possible to produce a tube whose internal shape is accurately calibrated. More particularly it is found to be almost impossible to produce a tube having an internal shape which is irregularly polygonal or similarly noncircular.